6th IRAM 30m Summer SchoolStar formation near and far

A. FuenteObservatorio Astronómico Nacional (OAN, Spain)

Photon Dominated RegionsI. Physical conditions

Photon Dominated Regions (PDRs)

Photon dominated regions or photodissociation regions are regions where the FUV radiation dominate the energetic balance and chemistry. i. The regions close to the O and B stars: HII regions, reflection nebulaeii. The planetary nebulae formed at the end of the life of low mass starsiii. The border (outer layer) of molecular cloudsiv. Diffuse cloudsv. The surface of molecular cloudsvi. The nucleus of star burst galaxiesvii. Distant galaxies?

Why to study PDRs?

Molinari et al. 2011, ApJ 735, L33

Why to study PDRs?

i. PDRs dominate the sky in the IR and far-IRii. In the PDRs occurs the interchange of energy between massive stars and the interstellar mediumiii. The comprehension of the physical and chemical processed occuring in PDRs are necessary to evolution of molecular clouds, and eventually of the galaxiesiv. Key objects such as protoplanetary disks of the hidden mass of galaxies are PDRs.

Mean interstellar UV field

Figure from “The Physics and Chemistry of the Interstellar Medium”, A.G.G.M. Tielens, ed Cambridge.

The stellar radiation field contains contributions from early-type stars, which dominate the FUV, A stars, which dominates the visible region and late-type stars, which are important at far-red to near-IR.The strength of the FUV interstellar field is expressed in terms of the Habing field = 1.2 x 10-4 erg cm-2 s-1 sr-1=1.6 x 10-3 erg cm-2 s-1=108 photons cm-2 s-1

Mean interstellar field = 1.7 x Habing field = Draine field.

Overview of a PDR

The physical /chemical conditions of a PDR depends on G0/n and Av

H/H2 transition at Av~2 mag

C+/C/CO transition at Av~4 magO/O

2 transition at Av~ 20 mag

G~G0 exp(-2 Av)

G0

Visual Extinction (Av)

R=3.1 diffuse ISMR=5.0 molecular clouds

R=3.1

Dust temperatureDust grains absorbed UV photons, their temperature increase and then irradiate at near-IR and far-IR. The dust temperature is given by the radiative balance

Energy absorbed by the grain = Energy emitted by the grain

In a PDR, the situation is a bit more complex because you also need to consider the emission from the other dust layers

Dust temperature

Gas Heating

1. UV radiation1.Photoionization of C atoms (Av < 4 mag)2.Photodissociation and collisional desexcitation of UV pumped

H2 (Av < 4 mag)3.Photoelectric effect on grains (Av < 4 mag)4.Gas-grain collisions5.Collisional desexcitation of the infrared pumped de of the

OI(63µm) line 2. Cosmic rays3. Shocks4. X rays

Photoelectric effect on grains

FUV photons (6eV<hυ<13.6eV) absorbed by a grain create energetic e-. If the energy of these e- is enough to overcome the work function of the grain and the Coulomb potential (in case of charged grains), the e- is injected into the gas with excess kinetic energy.

Figure from “The Physics and Chemistry of the Interstellar Medium”, A.G.G.M. Tielens, ed Cambridge.

Photoelectric effect on grains

The efficiency of this process is low (y=0.1): 96 % of the photons energy is absorbed by the grain (grain heating) and only 4% is used in the ejection of e-

The work funcion of a neutral grain is W = 6 eV. When a photon of 10 eV is absorbed by a grain and ejects an e-, only 4 eV are injected to the gas as kinetic energy.

Ratio between photon-ionization and recombination

Heating by ionization of C atoms

After photo-ionization, the e- is injected in the gas phase with a kinetic energy 3/2 k T

ion. In photodissociation regions, the H atoms

cannot be ionized and rhe first source of e- is C.

Photodissociation and collisional des-excitaion of UV pumper H

2

This heating mechanism can dominate when the density is very high, in this case

Gas-grain colisions

Γgd=2.0 10-33 n2 T1/2 (Td-T) erg cm-3 s-1

When the dust is warmer than the gas, this can be an important heating mechanism. In the contrary case, it is a cooling mechanism. This mechanism is more efficient is the dust is moving relative to the gas at high velocities. For high temperatures, the gas and grain are thermally coupled and are at the same temperature.

Cosmic rays

High energy protons (2 - 10 MeV) ionized tha gas (H2, He, HD) and inject energetic e- in the gas.

The efficiency depends on the gas composition, denstity and ionization degree

Cosmic rays can penetrate much deeper in the molecular clouds (until Av=100 mag) than UV photons (Av<10mag).

Gas cooling

Collisionally excited n>ncr T>

Eu/2 absorbing kinetic energy

from the gas

Decay radiatively emitting a photon hυThe photon removes the energy from the gas

hυ

hυ

Gas coolingWhen a transition of an atom or molecule is excited collisionally and desexcited adiatively, the gas loose energy and become cooler. The most important coolants are:

•Abundant

•With transitions that can be collisionally excited with the densities and temperatures of the cloud.

•It decays radiatively in a short time.

In the outer layers of the PDR (T>100 K and the gas is mainly atomic), the main coolants are CII (157µm) and OI (63µm and 145µm).

In molecular clouds, the main coolant is CO. In warm regions, water can be an important coolant.

Gas cooling (atomic lines)

In the case of optically thin lines, with the two-level aproximation the cooling rate is

Collisional coefficient between the two levels

In the low density limit, n<<ncr, essentially all collision is followed

by a spontaneous desexcitation emitting a photon and

In the case of optically thin lines, with the two-level aproximation the cooling rate isIn the case of optically thin lines, with the two-level aproximation the cooling rate is

Gas cooling (CO)

The CO lines are usually optically thick. A modeling of the excitation and radiative transfer is needed to estimate accurately the cooling rate. However, there are some analytical approximations such as

For 10K < T < 60 K y 102 < nH2 < 105 cm-3

Λ=1.2 10-23 n30.4 T30

0.5+(log n)/2 erg cm-3 s-1

n3=nH2/103 cm-3 y T30=T/30 K

(Goldsmith & Langer 1978,ApJ 222,881)

Example of Dense photodissociation region(Tielens & Hollenbach, 1985, ApJ 291, 722)

Example of Low Density photodissociation region(Hollenbach, Takahashi & Tielens 1991, ApJ 377, 192)

Gas and Dust temperature

Dense PDR Low density PDR

PAHs

PAHs are large molecules (number of C between 20 and 100) that share properties with molecules and (chemical reactions, fluoresent emission) and grains (can be formed by destruction of larger grains).

PAHs

PAHsPAHs influence the physics and chemistry of PDRs:1.- Heating the gas by the photoelectric effect2.- UV photons ionizes PAHs increasing the number of e-3.- Photochemistry

Comparison with observations

The goal of PDR models is to derive the physical conditions of the PDR (n, G

0, T) from observations. FIR lines are adequate, because

they tell us about the energetic balance.

PDR diagnostic model diagrams

Hollenbach & Mckee, 1989, ApJ 342, 306

PDR diagnostic model diagrams

PDRs diagnostic diagrams based on line intensities have the problem of the unknown beam filling factor. Ratios between the intensities of two lines or between lines and FIR continuum are preferred.

PDR diagnostic model diagrams

ISO -LWS observationsof

Herbig Ae/Be stars by

Lorenzetti et al (1999)

PDR diagnostic model diagrams

Lorenzetti et al (1999) used the

[OI]63µm/[CII]157µm and

[OI]63µm/[OI]145µm intensity

ratios to derive the physical

conditions of the PDRs

associated with Herbig Ae/Be

stars based on ISO data.

PDR diagnostic model diagramsPDR diagmostic diagrams are useful to derive global properties. If the main heating mechanism is the photoelectric effect, heating efficiency depends on the grain charge which is itself governed by the paramter G

0 T1/2 /n

e.

Gas heating efficiency

Since the [CII] 158 μm and [OI 63 μm lines have different critical densities, their intensity ratio is a good measure of the density.

Observations of PAHs

The abundance of PAHs can be derived from the ratio of the flux in the IR emission features to the total far-IR continuum, f

IR, that

is the PAH efficiency heating.

PAH FUV absorption cross section, = 7 x 10-8 cm2 per C atom

For a prototypical dense PDR region like the Orion Bar, fIR

=0.13

and the fraction of C in PAH fc= 3.5 x 10-2.

Assuming a typical size of 50 C atoms, this would imply a PAH abundance of 2.7 x 10-7.

Spitzer data (PAHs and H2)

Berné et al. (2009), ApJ 706, L160

Bright, extended emission of the PAHs bands and H2 rotational

linesLayered structure expected in a PDR

Different PDRs around UCHII region (different physical and chemical conditions)

G0 and n

H estimates from PAHs and H

2

H2 rotational lines are thermalized for n>104 cm-3

The I6.2

/I11.3

ratio is tracing the UV field allow to

determine the [PAH+]/[PAH0] ratio and the UV field(Galliano et al. 2008).

PAHs in NGC 7023(Joblin et al. 2010)

PAHs in disks(Berné et al. 2009)

Analysis of the PAH emission in 12 circumstellar disks. The goal was to characterize the properties of these particles in disks and investigate the possibility of using the PAH emission as tracer of the physical conditions or evolution of disks.

PAHs in disks(Berné et al. 2009)

The PAH emission can be used to derive the properties of the star.

References

1.- “The Physics and Chemistry of the Interstellar Medium”, A.G.G.M. Tielens, de. Cambridge University Press

2.- Tielens, A.G.G.M. & Hollenbach, D. J., 1985, ApJ 291, 722

3.- Hollenbach, D. J., Takahashi, T., Tielens, A.G.G.M. 1985, ApJ 291, 722